Although the coordination of growth and differentiation is fundamental to the development of all metazoans, little is known about how developmental signals are communicated to the cell cycle machinery. The unit on cell cycle regulation uses Drosophila oogenesis as a model system to examine the developmental regulation of the cell cycle. Our current research is focused on two problems: The regulation of the endo cycle and the relationship between cell cycle regulation and oocyte differentiation. During Drosophila oogenesis both germ-line derived nurse cells and somatic follicle cells enter the endo cycle and become polyploid. In the endo cycle, cells undergo successive rounds of DNA replication without an intervening mitosis. One goal of the laboratory is to understand how cells enter and maintain the endo cycle in response to specific developmental cues. Towards this end, we have designed a genetic screen, based on the FLP/FRT site specific recombinase system, to identify and characterize genes that regulate the endo cycle. This screen provides a general tool for the genetic dissection of pathways that control cell cycle regulation in the ovary. We are currently characterizing several mutants identified in a large scale FLP/FRT screen of the second chromosome. In addition to our studies of the endo cycle, we are examining how cell cycle regulation influences cell fate during oogenesis. We have determined that mutations in the twin gene alter both oocyte differentiation and cell cycle regulation in ovarian germ-line cysts. Our phenotypic analysis of twin mutants, indicate that characterization of the twin gene may further our understanding of both cell cycle control and cell fate decisions in the germ line. In the last year we have undertaken a detailed molecular and genetic characterization of the twin locus. - Drosophila, Oogenesis, Cell Cycle, Germ Line, Egg